Lasers need lenses no more

PORTLAND, Ore.  Focusing semiconductor lasers usually requires bulky optical lenses acting as a "collimator." Researchers have now demonstrated a plasmonic collimator that utilizes grooves etched directly into the semiconductor laser facet. If the technique is adopted -- Harvard University has applied for a patent on the process -- then semiconductor lasers can be downsized to a bare die without a lens.

Eventually the team at Harvard and Hamamatsu Photonics (Hamamatsu City, Japan) hope to demonstrate electrically-controlled polarization of laser beams for use in spintronics and quantum computing.

"The plasmonic collimator is applicable to all semiconductor lasers," claimed Harvard professor Federico Capasso. "This research opens up using plasmonic structures built on the laser facet to make any desired state of polarization -- the holy grail of spintronics and quantum information processing."

A quantum cascade laser patterned with a plasmonic collimator reduces beam divergence, Harvard University and Japanese researchers found.

The researchers claimed that if they can achieve a fully collimated laser, it will lower the cost of communications lasers by eliminating the need for lenses while enabling long-range chemical sensing. The latter could be used for homeland security and environmental monitoring applications.

The metallic facet that reflects light from semiconductor lasers does not perfectly collimate the beam, with a divergence of as much as 25 degrees. A relatively large external lens is required to refocus the laser into parallel beams of light. The researchers discovered that by etching a periodic array of sub-wavelength grooves into the facet, surface plasmons, or electromagnetic waves, are produced. The waves scatter the divergent beams so that they interfere constructively and re-emerge in parallel with the main beam.

The researchers claimed that the plasmonic collimator effect is similar to the way phased-array antennas steer a beam, creating the possibility of steering laser beams with no moving parts.

The researchers next hope to control the spatial emission pattern of semiconductor lasers, enabling laser beams to electrically steered over a wide angle without mirrors, prisms or lenses.

The current prototype used parallel line grooves that showed the technique could collimate a laser in the direction of the polarization of the laser. The researchers plan to etch concentric circular grooves in an attempt to fully collimate the laser beam in all directions.

Funding for the laser research was provided by Harvard's Center for Nanoscale Systems, the National Nanotechnology Infrastructure Network and the Air Force Office of Scientific Research.

What a great concept.
Have they thought about etching an array with a sinc (ie sin(x)/x) distribution similar to what is in the W mode fibre. The transform a sinc function often results in a step function in the transformed domain.